How to make an exact loop cut on an oblique edge?
Problem solved:

Subdivide edge one time.
With Edit Mesh Tools add-on set edge length (before this you need to apply scale by Ctrl+A&gt; Apply&gt; Apply all transforms or (in 3d Viewport) Object&gt; Apply&gt; Apply all transforms).
Switch to any orthographic view (top, bottom, left or other) and extrude vertex by pressing Ctrl and enabled (in 3d Viewport in Edit Mode) Options&gt; Auto Merge&gt; Split Edges and faces (on top).
Do this with all sides, removing duplicates if needed. You can check edge length by using Overlays&gt; Measurement&gt; Edge Length (in the 3D Viewport).

You can do the same thing with the Knife tool, but the Knife won't work in all situations.
Also, one of the ways:
Bevel edges with settings: segments - 2, profile - 1. Bevel has differrent modes (Offset, Width and other), we need Offset. For bevel modifier you can do vertex groups where you need and bevel just them.